Metal strip resistor for mitigating effects of thermal EMF

ABSTRACT

A metal strip resistor includes a resistor body having a resistive element formed from a strip of an electrically resistive metal material and a first termination electrically connected to the resistive element to form a first junction and a second termination electrically connected to the resistive element to form a second junction, the first termination and the second termination formed from strips of electrically conductive metal material. The resistive element, the first termination, and the second termination being arranged mitigate thermally induced voltages between the first junction and the second junction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/161,636 filed on Mar. 19, 2009 and U.S. Provisional ApplicationSer. No. 61/169,377 filed on Apr. 15, 2009, both of which areincorporated by reference as if fully set forth.

FIELD OF THE INVENTION

The present invention relates to resistors. More specifically, thepresent invention relates to metal strip resistors configured to assistin mitigating the effects of thermal EMF.

BACKGROUND OF THE INVENTION

Thermal electromotive force (EMF) is a voltage that is generated whentwo dissimilar metals are joined together. When there are two of thesejunctions that are of opposite polarity and the temperature of thejunctions are equal, there is no net voltage. When one of the junctionsis at a different temperature than the other, a net voltage differencecan be detected. A resistor may have a metal resistive element connectedbetween copper terminals, thereby providing two junctions and making theresistor susceptible to adverse effects of thermal EMF.

Resistors of this construction are often used to sense current bymeasuring the voltage drop across the resistor. In cases where thecurrent is low, the signal voltage generated across the resistor is alsovery small and any voltage caused by thermal EMF can cause a significantmeasurement error.

One prior art approach to addressing this problem has been to change themetal alloy used for the resistive element to one with a lower thermalEMF. In some cases this presents other challenges such as increasedcost, an increase in bulk resistivity that creates a resistor geometrythat is costly to manufacture, or sacrifices other electricalcharacteristics such as TCR (temperature coefficient of resistance).

Another prior art approach has been to add an ASIC (application specificintegrated circuit) that is programmed to compensate for the offsetvoltage created by the thermally induced EMF. Such an approach addsmaterial cost, complexity to the assembly, and manufacturing cost interms of assembly steps and equipment.

What is needed is to provide a resistor that mitigates the effects ofthermal EMF while not imposing constraints on the type of metalresistance alloy used.

SUMMARY OF THE INVENTION

According to one embodiment a metal strip resistor is provided. Themetal strip resistor includes a resistor body having at least oneresistive element formed from a strip of a resistive metal material,(such as Evanohm, Manganin, or others), and a first terminationelectrically connected to the resistive element to form a first junctionand a second termination electrically connected to the resistive elementto form a second junction; the first termination and the secondtermination being formed from strips of highly electrically conductivemetal material, such as copper or others, with high electricalconductivity. Prior art metal strip resistors are described in U.S. Pat.No. 5,604,477 (Rainer et al.). The resistive element, the firsttermination, and the second termination are arranged to assist inmitigating effects of thermally induced voltages between the firstjunction and the second junction. The resistor body may include a foldbetween a first portion of the resistor body and a second portion of theresistor body. A thermoconductive and electrically non-conductivematerial may be used to thermally connect the first portion of theresistor body to the second portion of the resistor body and assist inreducing the temperature differential between the first junction and thesecond junction to thereby mitigate the effects of the thermally inducedvoltages between the first junction and the second junction.

According to another embodiment, a metal strip resistor is provided. Themetal strip resistor includes a resistor body having a resistive elementformed from a strip of a resistive metal material and a firsttermination joined to the resistive element to form a first junction anda second termination joined to the resistive element to form a secondjunction; the first termination and the second termination being formedfrom strips of highly electrically conductive metal material. Theresistor body is folded onto itself and mating surfaces are bonded witha thermally conductive and electrically non-conductive adhesive tothereby equalize the temperature between the two sides of the resistorbody thus mitigating effects of thermally induced voltages between thefirst junction and the second junction.

According to another embodiment, a metal strip resistor includes aresistor body having a resistive element formed from a strip of aresistive metal material and a first termination joined to the resistiveelement to form a first junction and a second termination joined to theresistive element to form a second junction; the first termination andthe second termination being formed from strips of highly electricallyconductive metal material. The resistive element, the first termination,and the second termination are arranged to provide a first temperaturegradient along a length of the first junction and a second temperaturegradient along a length of the second junction such that thetemperatures at any two adjacent points on opposite junctions aresubstantially equal.

According to another embodiment, a method of manufacturing a metal stripresistor includes joining a resistive metal material with anelectrically conductive material to form a resistor body with aplurality of junctions between the resistive metal material and theelectrically conductive material, folding the resistor body, and bondingthe resistor body on one side of the fold to the resistor body on anopposite side of the fold with a thermoconductive and electricallynon-conductive adhesive to thereby form a metal strip resistorconfigured for mitigating effects of thermally induced voltages.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a metal strip resistor prior to folding;

FIG. 2 illustrates a metal strip resistor prior to folding with a dualresistive element;

FIG. 3 illustrates the metal strip resistor of FIG. 1 after folding;

FIG. 4 illustrates the metal strip resistor of FIG. 2 after folding;

FIG. 5 is a cross sectional view of the metal strip resistor of FIG. 3;

FIG. 6 is a cross sectional view of the metal strip resistor of FIG. 4;

FIG. 7 illustrates a resistor with a geometry for mitigating effects ofthermally induced voltages by maintaining an equal temperature gradientalong each junction thus equalizing the temperature differential acrossthe resistive element at any two adjacent points on opposite junctions;

FIG. 8 illustrates another resistor with a geometry for mitigatingeffects of thermally induced voltages by maintaining an equaltemperature gradient along each junction thus equalizing the temperaturedifferential across the resistive element at any two adjacent points onopposite junctions;

FIG. 9 illustrates another resistor with a geometry for mitigatingeffects of thermally induced voltages by maintaining an equaltemperature gradient along each junction thus equalizing the temperaturedifferential across the resistive element at any two adjacent points onopposite junctions;

FIG. 10A-10D illustrates another metal strip resistor for mitigatingeffects of thermally induced voltages; and

FIG. 11A-11D illustrates another metal strip resistor for mitigatingeffects of thermally induced voltages.

DETAILED DESCRIPTION

The embodiments disclosed herein provide a resistor for mitigatingeffects of thermal electromotive force (EMF). This allows the use of anynumber of types of metal resistance alloy regardless of thermal EMF andnegates any termination to termination temperature differential. Theembodiments disclosed herein achieve desirable results by usingappropriate resistor geometries, metal forming, and/or heat transfermaterials.

Note that, rather than change a resistor's resistive element materialand/or termination material, or add compensation circuitry to offset thethermal EMF of a specific set of resistor metal alloys, the embodimentsdisclosed herein provide for using a geometry that brings both metallicjunctions to the same temperature. In overcoming the problem in this waythe embodiments disclosed herein function regardless of the metal alloysused and their specific thermal EMF characteristics. Thus, theembodiments disclosed herein are not limited to particular types ofmaterials and materials may be selected to optimize other electricalcharacteristics such as TCR, resistance, or stability without concernfor the thermal EMF. This is a significant advantage.

FIG. 1 illustrates a metal strip resistor 10 with a resistor body 11prior to folding. The resistor body 11 has a first termination 16 and asecond termination 20. The resistor body 11 includes at least oneresistive element 13. The first termination 16 and the secondtermination 20 comprise metal strips. The resistive element 13 alsocomprises a metal strip of a different alloy than the termination metal.The strips are joined to provide for electrical and mechanicalconnections between the first termination 16 the second termination 20and the resistive element 13. A first junction 15 is provided where thefirst termination 16 is joined to the resistive element 13 and a secondjunction 17 is provided where the second termination 20 is joined to theresistive element 13.

A fold line 12 is shown at the midpoint which is substantiallyequidistant between each end of the resistor body 11 and which extendsthrough a mid point of the resistive element 13 such that a firstresistive element portion 14 and a second resistive element portion 18of the resistive element 13 are on opposite sides of the fold line 12,and such that the first termination 16 and the second termination 20 areon opposite sides of the fold line 12 and the first junction 15 and thesecond junction 17 are on opposite sides of the fold line 12. Theresistor body 11 is subsequently folded on a line 12 which issubstantially equidistant from each end of the resistor body 11. It isunderstood that the fold line can be located at various locations alongthe resistor body other than the midpoint.

Prior to folding, one half of what will be the inside of the foldedresistor is coated with a material that has good thermal conductivityyet is not electrically conductive (thermally conductive material). Thethermally conductive material can also include an adhesive that willbond the two halves of the resistor body together. FIG. 3 and FIG. 5illustrate the resistor after folding and bonding. The resistor body isfolded in half onto itself. As shown in FIG. 5, there is a gap 22between the halves. The gap 22 may have a size in the range of 0.001inch (0.0254 mm) to 0.005 inch (0.127 mm), although the gap may belarger or smaller. The gap 22 is filled with a thermally conductivematerial or adhesive 30 such as a material which includes an elastomerand a thermally conductive filler. Other thermally conductive materialscould be used to achieve the desired objectives of bonding and thermaltransfer from one half to the other while electrically insulating onehalf from the other.

By thermally connecting each half of the resistor 10 in this manner thetemperature of each of the two copper-to-resistive alloy junctions areheld at equal temperatures thus negating any net voltages from thethermal EMF of the junctions. Thus, the thermally conductive material 30allows heat to be transferred between opposite sides of the resistor sothat the first junction and the second junction are held atsubstantially equal temperatures to thereby mitigate effects of thermalEMF.

Another embodiment is shown in FIGS. 2, 4 and 6. The resistor of FIGS.2, 4 and 6 is the same as the resistor of FIGS. 1, 3 and 5 except thatthe resistive element 13 is a dual resistive element such that the firstportion 14 is separated from the second portion 18 by a highlyelectrically conductive metal material 24. Note that in FIG. 2 there arejunctions 15A, 15B on opposite sides of the first portion 14 of theresistive element 13 and there are junctions 17A, 17B on opposite sidesof the second portion 18 of the resistive element 13. As best shown inFIG. 6, the dual resistive element allows for the conductive material 24to be in the center of the folding line 12 so that mechanical stress isnot induced into the resistive element 13. This configuration assists inpreventing possible resistance problems which may occur if the fold lineis through the resistive element. Although this configuration has fourjunctions 15A, 15B, 17A, 17B, instead of two, there are oppositejunctions at each of the two possible temperatures. Thus, thisconfiguration still results in mitigation of thermal EMF.

FIGS. 10A-10D illustrate another embodiment similar to that shown inFIG. 1. FIG. 10D illustrates the resistor body 11 prior to folding. Notethat the geometry of the unfolded resistor body 11 is similar to theshape in FIG. 1, except that the second termination has a notch 26 inits outer edge to assist in folding into the configuration best shown inFIG. 10B.

FIGS. 11A-11D illustrate another embodiment of a resistor shows aresistor element which uses less welded strip by eliminating theterminal protrusions yet uses the same method of forming and bonding themetal junctions to prevent any junction temperature differentials.

FIG. 7, FIG. 8 and FIG. 9 show other examples of resistor geometriesthat provide for mitigating effects of thermal EMF associated withjunctions, but without using folding. Each is of the metal stripresistor construction. Each of the copper (or otherconductor)-to-resistive alloy junctions in any of these designs may havea temperature gradient along the length of each junction caused by anypossible temperature differential between the two terminals. As shown inFIGS. 7 and 8, the resistor body 11 can include electrically conductiveportions that are generally tapered or triangular in shape. Since thetemperature gradient along the length of each junction is the sameregardless of which side of the resistive element, the temperature atany two adjacent points on opposite junctions is substantially equal,and each junction is of an opposite polarity, thus thermally inducedvoltages are equal and opposite cancelling each other out. Note thatvarious configurations are contemplated for mitigating thermal EMF inthis manner.

Therefore, a metal strip resistor for mitigating the effects of thermalEMF has been disclosed. The embodiments disclosed herein provide aresistor for mitigating effects of thermal EMF. The embodimentsdisclosed herein allow the use of any number of types of metalresistance alloy regardless of thermal EMF and negates any terminal toterminal temperature differential. The embodiments disclosed hereinachieve desirable results by using appropriate resistor geometries,metal forming, and/or heat transfer materials. The present inventioncontemplates numerous variations, options, and alternatives includingvariations in the geometry used, the types of materials used, andothers.

1. A resistor comprising: a first termination and a second termination;a body having at least one resistive element, the body having a firstend coupled to the first termination to form a first junction and asecond end coupled to the second termination to form a second junction;wherein the body is folded onto itself defining a gap, the firsttermination and second termination being disposed on opposite sides ofthe gap; and a thermally conductive material disposed in at least aportion of the gap.
 2. The resistor of claim 1 wherein the thermallyconductive material thermally connects the first and second junction. 3.The resistor of claim 1 wherein the body has a single resistive element.4. The resistor of claim 3 wherein the body is folded through theresistive element wherein the resistive element has a first resistiveelement portion disposed on one side of the gap and a second resistiveelement portion disposed on an opposite side of the gap.
 5. The resistorof claim 4 wherein the gap is disposed between the first resistiveelement portion and the second resistive element portion, wherein thethermally conductive material thermally connects the first resistiveelement portion and the second resistive element portion.
 6. Theresistor of claim 1 wherein the body has a plurality of resistiveelements.
 7. The resistor of claim 1 wherein the body has first andsecond resistive elements.
 8. The resistor of claim 7 wherein the bodyis folded through a point located between the first and second resistiveelement wherein the first resistive element is disposed on one side ofthe gap and the second resistive element is disposed on an opposite sideof the gap, wherein the thermally conductive material thermally connectsthe first resistive element and the second resistive element.
 9. Theresistor of claim 1 wherein the thermally conductive material furthercomprises an adhesive.
 10. The resistor of claim 1 wherein the thermallyconductive material is electrically non-conductive.
 11. The resistor ofclaim 1 wherein the first termination and the second termination arecomprised of strips of electrically conductive metal material.
 12. Theresistor of claim 1 wherein the first termination and the secondtermination are comprised of copper.
 13. The resistor of claim 1 whereinthe body is folded onto itself and bonded with a thermally conductiveadhesive thereby mitigating thermally induced voltages between the firstjunction and the second junction.
 14. The resistor of claim 1 whereinthe body is folded at its midpoint.
 15. A method of manufacturing aresistor, comprising: joining a first end of a body to a firsttermination forming a first junction and joining a second end of thebody to a second termination forming a second junction, wherein the bodyincludes at least one resistive element; folding the body onto itself,forming a gap, the first termination and second termination beingdisposed on opposite sides of the gap; and applying a thermallyconductive material in at least a portion of the gap.
 16. The method ofclaim 15 wherein the thermally conductive material thermally connectsthe first and second junction.
 17. The method of claim 15 wherein thebody has a single resistive element.
 18. The method of claim 15 whereinthe body is folded through the resistive element wherein the resistiveelement has a first resistive element portion disposed on one side ofthe gap and a second resistive element portion disposed on an oppositeside of the gap.
 19. The method of claim 18 wherein the gap is disposedbetween the first resistive element portion and the second resistiveelement portion, wherein the thermally conductive material thermallyconnects the first resistive element portion and the second resistiveelement portion.
 20. The method of claim 15 wherein the body has aplurality of resistive elements.
 21. The method of claim 15 wherein thebody has first and second resistive elements.
 22. The method of claim 21wherein the body is folded through a point located between the first andsecond resistive element wherein the first resistive element is disposedon one side of the gap and the second resistive element is disposed onan opposite side of the gap, wherein the thermally conductive materialthermally connects the first resistive element and the second resistiveelement.
 23. The method of claim 15 wherein the thermally conductivematerial further comprises an adhesive.
 24. The method of claim 15wherein the thermally conductive material is electricallynon-conductive.
 25. The method of claim 15 wherein the first terminationand the second termination are comprised of strips of electricallyconductive metal material.
 26. The method of claim 15 wherein the firsttermination and the second termination are comprised of copper.
 27. Themethod of claim 15 wherein the body is folded onto itself and bondedwith a thermally conductive adhesive thereby mitigating thermallyinduced voltages between the first junction and the second junction. 28.The method of claim 15 wherein the body is folded at its midpoint.
 29. Aresistor comprising: a first termination and a second termination; abody having at least one resistive element, the body having a first endcoupled to the first termination to form a first junction having alength and a second end coupled to the second termination to form asecond junction having the same length; wherein the resistive element,the first termination, and the second termination are arranged to have atemperature gradient along the length of each junction, mitigatingthermally induced voltages between the first junction and the secondjunction.
 30. A method of manufacturing a resistor, comprising: joininga first end of a body to a first termination forming a first junctionhaving a length and joining a second end of the body to a secondtermination forming a second junction having the same length, whereinthe body includes at least one resistive element; wherein the resistiveelement, the first termination, and the second termination are arrangedto have a temperature gradient along the length of each junction,mitigating thermally induced voltages between the first junction and thesecond junction.